This invention relates to blanks for thread-forming fasteners and thread-forming fasteners which are formed from such blanks.
It is extremely advantageous to provide a thread-forming fastener because of the resultant mechanical and material characteristics of the fastener connection. More particularly, a thread-forming fastener typically produces a more reliable threaded connection between the fastener and workpiece because the material is deformed and not removed from an inside surface of a hole formed through the workpiece.
In automated fastener insertion operations, it is desirable to provide a thread-forming fastener which minimizes required initial driving effort for thread-forming and maintains proper alignment of the fastener with the workpiece pilot hole. A general problem exists with typical thread-forming fasteners in that they easily become misaligned because their forming features connect with the workpiece pilot hole in a plane that approximates to the helix angle of the screw thread. As the forming feature follows the helix spiral of the screw thread, the workpiece internal thread will be axially misaligned to the design requirements of the assembly application. Furthermore, cross-threading may occur when initially starting a thread-forming fastener into a threaded hole whereby a minor shift in either the fastener or the workpiece may misalign the threads on the fastener and those formed in the threaded hole. Misaligned or crossed thread results in reducing the strength, reliability and quality of the fastener connection to the workpiece. For these reasons, thread-forming fasteners which don't provide cross-threading resistance features have not been as successful in automated installation operations.
As intimated above, self-alignment as well as cross-threading resistance features on a thread-forming fastener are also important when the fastener is removed from and substantially replaced or reinserted into the workpiece in which it formed threads. The design of prior art thread-forming fasteners promote further thread-forming even in a previously threaded hole, when the fastener threads are misaligned with the hole threads. For example, when a thread-forming fastener is removed from a hole in which it has formed threads, cross-threading may occur upon reinsertion of the fastener into the hole resulting in stripping or eliminating the threads inside of the hole. Stripped or eliminated threads in the hole provide little or no mechanical interface to retain the fastener in the workpiece thereby substantially degrading the quality of the fastener connection to the workpiece.